In the 1990s, the fly Philornis downsi was accidentally introduced to the Galapagos Islands, probably in a shipment of fruit. The adult flies are harmless enough as invaders go, but their kids are a real problem for the islands’ native birds, some of which are found nowhere else in the world and a few of which are endangered.

The flies lay their eggs in birds’ nests just as the chicks are emerging from their own eggs, and once the larvae hatch, they begin to eat their hosts from both inside and out. No larger than grains of rice, the maggots wriggle their way into the baby birds’ nostrils and eat away at their nasal cavities. As they get larger, the parasites burst back out and continue to live in the nest, hiding by day and emerging each night to suck blood from the chicks. This is often too much for the birds to bear. In some years, the parasites have killed every single chick in a given area and caused every nest to fail. Even if the birds survive, they often have difficulty eating because their beaks are deformed from the larvae that crawled around inside.

Among the birds that the flies maim and kill are Darwin’s finches, a group of 15 related species whose beaks diverged in form as adaptations to their function and were a key piece of evidence for Charles Darwin when he was developing his idea of evolution by natural selection. And even though they’re textbook examples of adaptation, the birds haven’t adapted to the parasites yet because they don’t have a long evolutionary history with them. For now, it falls to scientists to protect them. Conservation biologists have tried treating the nests with insecticides, which increases the number of chicks that survive long enough to fly on their own, and also removing chicks of the most vulnerable species from their nests and raising them in incubators.

Neither of these strategies is inexpensive or easy. Treating the nests is particularly tricky because they’re often hard to find or placed too high in the trees to reach. Now a team of biologists thinks they’ve found a new, more efficient way to do it: lend the birds the insecticides and let them be their own exterminators.

The idea came to University of Utah doctoral student Sarah Knutie as she watched finches come to the laundry lines outside her island dorm and pull threads from clothes and towels to add to their nests. She wondered if the birds would also take fibers that had been treated with permethrin—an insecticide often used in flea collars and lice shampoos—and work them into the nests to “self-fumigate” them.

To find out, Knutie, other students and their advisor Dale Clayton fashioned 30 dispensers out of wire mesh, filled them with either permethrin- or water-soaked cotton, and placed them along a road near nesting sites on Santa Cruz Island.

When the breeding season was over and the baby birds left home, the researchers collected and dissected 26 empty nests built by four different finch species. Twenty-two of the nests contained cotton from the dispensers, and more than half of those contained the insecticide-laced cotton.

The birds were apparently happy to take the dosed cotton, and it paid off for them. The nests with the permethrin cotton in them contained about half as many parasites as the ones that had plain cotton or no cotton at all, and all but one of the nests with a least of a gram of treated cotton—about a thimble's worth—were parasite-free.

If more cotton dispensers can be installed and maintained, they could make a huge difference for some of the islands’ birds. One of Darwin’s finches, the mangrove finch, has a population of less than 100 birds confined to about a square kilometer of land. It would only take 60 dispensers, Knutie says, to protect the whole population from the flies.

The researchers hope that the same method could be used to protect other birds and nest-building animals against parasites and pests, from Hawaiian honeycreepers dealing with feather lice to prairie dogs that are literally plagued by Yersinia pestis-carrying fleas. Just a little puff of cotton could go a long way in helping these animals help themselves.